Field of the Invention
The invention relates to a low-alloyed steel having excellent processability and scaling resistance, as well as components and parts made thereof In particular, the present invention is directed to steel for forming parts which provides the formed parts with good scaling resistance, as well as components made thereof Low-alloyed steels are steels in which no alloying element exceeds a median content of 5 mass percent.
Description of Related Art
Steel alloys are defined according to the following rules: Because iron (Fe) is well-known to comprise the majority of the alloy, it is typically left-out of the formula. The first position indicates the carbon content in mass percent multiplied by 100, followed by the chemical symbols of the alloying elements in the order of decreasing mass fractions, and at the end, in the same order and separated by hyphens, the mass fractions of the previously indicated alloying elements, which are multiplied by the following factors in order to arrive at larger integers:
×1000: B
×100: C, N, P, S, Ce
×10: Al, Cu, Mo, Ti, V, Be, Ta, Zr, Nb, Pb
×4: Cr, Co, Mn, Ni, Si, W.
In all instances Fe comprises the remainder of the alloy. In cases where an alloying element is present, but at amounts which are not meaningful, the number referring to their content can be omitted.
Because steels with particularly low carbon content have excellent processing properties, more recently they have been used extensively for forming parts, especially for vehicles, machine engineering, construction of large engines etc. Workpieces for forging are usually obtained by decarbonisation of molten steel, which has been produced by a converter etc., where e.g. a vacuum-degassing method, such as the Ruhrstahl-Haereus (RH) method, is used in order to lower the carbon concentration to a particularly low level. Afterwards, usually continuous casting is performed. For forming applications, 42CrMo4 IM (inclusion modified) steel or 43 CrMo4 has often been used as low-alloyed steel.
Conventional 42CrMoS4 IM steel has the following composition:
ChemicalMinMaxcomposition(wt. %)(wt. %)C0.380.45Si0.40Mn0.700.90P0.035S0.035Cr0.901.20Mo0.150.30
In the hardened and tempered state, 42CrMo4 IM steel has a tensile strength of 900 to approximately 1200 MPa, and a yield strength of at least 650 MPa. This steel has the following advantages: Inclusions are less abrasive, acting like lubricants and barriers at the contact points of tool and workpiece. Compared to the standard class of the IM steels, they already result in                better cutting properties with reduced processing costs        up to 30% longer lifetime for a specific cutting speed        up to 30% higher cutting speeds for a specific lifetime.        
The alloying components of the steel used in the known alloy have the following effects, among others.
Carbon lowers the melting point and increases the hardness and tensile strength through formation of Fe3C. In higher amounts it increases the brittleness and lowers the forgeability, weldability, fracture strain and notch impact strength. Likewise, the malleability is lowered when carbon is added in higher amounts and it must therefore be added in lower amounts.
Chrome lowers the critical cooling rate and increases wear resistance, high temperature strength, and scaling resistance. The tensile strength is increased, as chrome acts as a carbide binder. From 12.2 wt. % and above, it increases corrosion resistance (stainless steel) and has a ferrite-stabilizing effect. Unfortunately, it lowers notch impact strength and weldability and decreases thermal and electric conductivity. Through the addition of chrome, the best results in effective hardness and hardness penetration are achieved.
Molybdenum enhances hardenability, tensile strength and weldability. Unfortunately, it decreases ductility and malleability. Molybdenum also increases hardening properties and advantageously complements chrome. In addition, Mo enhances the high temperature strength as well as tempering resistance, a property which is especially important when it comes to tempering.
Sulphur increases machinability, but lowers the ductility and thus the malleability of the iron alloy.
A conventional heat treatable steel 41CrS4, which is used for the same purposes, consists of:
ChemicalMinMaxcomposition(wt. %)(wt. %)C0.380.45Mn0.600.90Si0.40S0.40Cr0.901.2
The heat-treatable steel 41CrS4 is a versatile material and is mainly used in automotive engineering and vehicle construction. It is used for components for which strength requirements are not as high as for parts made of the heat-treatable steel 42CrMo4. 41CrS4 is hot-formed at 850° C. to 1310° C. and slowly cooled-down afterwards.
As 41CrS4 is hard to weld, it should not be used in welded constructions. In the hardened and tempered condition, 41CrS4 steel has a yield strength of 560 to 800 MPa and a tensile strength of 950 to 1200 MPa at room temperature.
The conventional steels 42CrMo4 and 41CrS4 are very versatile. With the described properties, the materials are suitable for high as well as extremely high dynamic stress and static loads. They are applied based on the required strength and ductile values. However, the dimensioning of components and parts always has to be considered. Especially in hot and cold forming processes, these steels have excellent mechanical machinability so that they are widely used in vehicle construction, machine engineering, construction of large engines etc. However, they do not have sufficient scale resistance for some applications (highly thermally-strained parts) and are also not sufficiently strong for light-weight steel construction.
Due to stricter environmental legislation, especially in the USA, in order to achieve a reduction of pollutants in exhaust gases, the pressures and consequently also the temperatures in the combustion chamber of diesel engines had to be increased. According to the new and stricter conditions for Ferrotherm pistons, the temperatures in the combustion chamber could reach up to about 500° C., with the temperatures probably being a little lower in the interior of the piston.
Aluminium alloys which have often been used in cars are less and less able to meet the necessary load increases. Here, a two-part solution presented itself, consisting of a highly loaded piston head part and the piston skirt. As a standard material for the piston head, the material 42CrMo4 is often selected in a tempered version. The strength of these components lies between about 870 and about 1080 MPa. The high temperature strength, alternating load resistance, thermal shock stability and oxidation resistance of these heat-treatable steels are also just sufficient for the present conditions.
Because scale resistance needs to be improved for the new applications and also with view to the high prices for these conventional steels, which particularly arise through addition of Mo, the creation of steel with better mechanical properties is desired. So far it has been assumed that for:
up to 400° C., the use of unalloyed and manganese-alloyed steels is adequate;
up to 550° C., the use of Mo(—V)-alloyed steels is adequate;
up to 600° C., the use of scaling-resistant steels, which are high-alloyed with Cr is appropariate; and
for greater than 600° C., the use of high-alloyed, austenitic Cr—Ni steels is required. However, such high-alloyed steels are expensive.
Thus, only high-alloyed steels have been used as scaling resistant and high-temperature resistant steels, resulting in correspondingly high costs for the alloying elements.
Accordingly, one objective of the present invention is to enhance the scaling resistance of low-alloyed steels for highly thermally-stressed steel parts.